long long bit-fields

long long bit-fields

[Falk Hueffner]

Hi,

PR c/6346, middle-end/7018 and target/9620 deal with long long
bit-fields. I guess part of the confusion comes from the fact that the
C standard doesn't specify how they should behave (at least I couldn't
find it). So what is the following program supposed to print?

#include <stdio.h>

int
main(int argc, char *argv[]){
    struct {
	unsigned long long ull31a: 31;
	unsigned long long ull31b: 31;
    } x;
    x.ull31a = -1;
    x.ull31b = 1;

    printf("%016llx %016llx %016llx\n",
	   (x.ull31a << 48) >> 48,
	   (x.ull31a << 16),
	   (x.ull31b << 48));
}

Candidates are:

ffffffffffffffff ffffffffffff0000 0000000000010000 (gcc)
ffffffffffffffff 00007fffffff0000 0001000000000000 (Compaq C)
000000000000ffff 00007fffffff0000 0001000000000000 (g++)
0000000000000000 ffffffffffff0000 0000000000000000 (Compaq C++)

-- 
	Falk

Re: long long bit-fields

[Segher Boessenkool]

Falk Hueffner wrote:
> Hi,
> 
> PR c/6346, middle-end/7018 and target/9620 deal with long long
> bit-fields. I guess part of the confusion comes from the fact that the
> C standard doesn't specify how they should behave (at least I couldn't
> find it). So what is the following program supposed to print?
> 
> #include <stdio.h>
> 
> int
> main(int argc, char *argv[]){
>     struct {
> 	unsigned long long ull31a: 31;
> 	unsigned long long ull31b: 31;
>     } x;
>     x.ull31a = -1;
>     x.ull31b = 1;
> 
>     printf("%016llx %016llx %016llx\n",
> 	   (x.ull31a << 48) >> 48,
> 	   (x.ull31a << 16),
> 	   (x.ull31b << 48));
> }
> 
> Candidates are:
> 
> ffffffffffffffff ffffffffffff0000 0000000000010000 (gcc)

Correct (the integer promotions promote an unsigned integer
bit-field to a signed integer, if it fits).

> ffffffffffffffff 00007fffffff0000 0001000000000000 (Compaq C)

Not correct.

> 000000000000ffff 00007fffffff0000 0001000000000000 (g++)
> 0000000000000000 ffffffffffff0000 0000000000000000 (Compaq C++)

Don't know about C++, sorry.


Segher

Re: long long bit-fields

[Falk Hueffner]

Segher Boessenkool <segher@koffie.nl> writes:

> Falk Hueffner wrote:
> > PR c/6346, middle-end/7018 and target/9620 deal with long long
> > bit-fields. [...]
>
> Correct (the integer promotions promote an unsigned integer
> bit-field to a signed integer, if it fits).

Okay. Then we can close middle-end/7018. Most other bit-field bugs
seem to be caused by casts to the bit-field's type getting discarded
erraneously, by the folder presumably. I'll try to have a look at that
some time in the future.

How about:

    struct { unsigned long long ull31a: 33; } x = { -1 };
    printf("%016llx\n", (x.ull31a << 48) >> 48);

We don't currently promote to signed long long if it fits. Should we?

-- 
	Falk

Re: long long bit-fields

[Segher Boessenkool]

Falk Hueffner wrote:
> How about:
> 
>     struct { unsigned long long ull31a: 33; } x = { -1 };
>     printf("%016llx\n", (x.ull31a << 48) >> 48);
> 
> We don't currently promote to signed long long if it fits. Should we?

Yes.  But IMHO, of course.


Segher